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Moons of Saturn
Saturn has 61 moons with confirmed orbits, 52 of which have names, and most of which are quite small. There are also hundreds of known "moonlets" embedded within Saturn's rings. With seven moons that are large enough to be rounded in shape (and which would thus be considered dwarf planets if they were in direct orbit about the Sun) in addition to the planet's broad and dense rings, the Saturnian system is the most diverse in the solar system. Particularly notable are Titan, the second largest moon in the Solar System, with an earth-like atmosphere and a landscape including hydrocarbon lakes and river networks, and Enceladus, which may harbor liquid water under its south pole. Twenty-three of Saturn's moons are regular satellites, with prograde orbits that are not greatly inclined with respect to Saturn's equatorial plane. In addition to the seven major satellites, an additional four moons are small trojans that share an orbit with a larger moon, and two more are mutually co-orbital moons. Finally, two moons are known to orbit within gaps in Saturn's rings. The regular satellites are traditionally named after Titans or other figures associated with the mythological Saturn. The remaining thirty-eight, all small, are irregular satellites, whose orbits are much farther from Saturn, have high inclinations, and are mixed between prograde and retrograde. These moons were likely captured minor planets, or debris from the breakup of such bodies after they were captured, creating collisional families. The irregular satellites have been classified by their orbital characteristics into Inuit, Norse, and Gallic groups, and their names are chosen from the corresponding mythologies. The rings of Saturn are made up of icy objects ranging in size from microscopic to hundreds of metres, each of which is on its own orbit about the planet. Thus a precise number of Saturnian moons cannot be given, as there is no objective boundary between the countless small anonymous objects that form Saturn's ring system and the larger objects that have been named as moons. At least 150 "moonlets" embedded in the rings have been detected by the disturbance they create in the surrounding ring material, though this is thought to be only a small sample of the total population of such objects. A confirmed moon is given a permanent designation by the IAU consisting of a name and a Roman numeral. The nine moons that were known before 1900 (of which Phoebe is the only irregular) are numbered in order of their distance from Saturn; the rest are numbered in the order by which they received their permanent designations. Eight small moons of the Norse group have not received a permanent designation. Characteristics The Saturnian moon system is very lopsided, with one moon, Titan, comprising more than 96 percent of the mass in orbit around the planet. The six other spherical moons constitute roughly four percent, while the remaining 54 together with the rings only comprise 0.01 percent. Many of Saturn's moons, such as Pan and Daphnis, orbit within Saturn's ring system and have orbital periods only slightly longer than the planet's rotation period. But several irregular satellites in the outermost regions of Saturn's moon system, in particular the Norse group, have orbital radii of millions of miles and orbital periods lasting several years. The moons of the Norse group also lie almost perpendicular to Saturn's equator, while the moons of the Gallic and Inuit groups have inclinations ranging from 30 to 50 degrees. The innermost moons and most regular satellites all have inclinations ranging from less than a degree to ~1.5 degrees. The exception is Iapetus, which has an inclination of 7.57 degrees. Discovery Before the advent of telescopic photography, eight moons of Saturn were discovered by direct observation using an optical telescope. Saturn's largest moon, Titan, was discovered in 1655 by Christiaan Huygens. Tethys, Dione, Rhea and Iapetus (the "Sidera Lodoicea") were discovered 1671-1684 by Giovanni Domenico Cassini. William Herschel discovered Mimas and Enceladus in 1789. Hyperion was discovered 1848 by W.C. Bond, G.P. Bond and Lassell. The use of long-exposure photographic plates made it possible to discover additional moons. The first to be discovered in this manner, Phoebe, was found in 1899 by W.H. Pickering. In 1966, the satellites Janus and Epimetheus were observed, but not confirmed, and it was not realized that there were two distinct moons sharing an orbit. Observations by spacecraft : Huge Titan and Dione at the bottom, small Prometheus (under the rings) and tiny Telesto above center]] The study of the outer planets has since been revolutionized by the use of unmanned space probes. The arrival of the Voyager space probes at Saturn in 1980 resulted in the discovery of seven additional moons, bringing the total from 10 to 17. Epimetheus was confirmed as distinct from Janus. In 1990, Pan was discovered in archival Voyager images. The Cassini mission, which arrived at Saturn in the summer of 2004, discovered three small moons in the inner Saturnian system (Methone, Pallene and Polydeuces) as well as three suspected but unconfirmed moons in the F Ring. On November 16, 2004 Cassini scientists announced that the structure of Saturn's rings indicates the presence of several more moons orbiting within the rings, but only one, Daphnis, has been visually confirmed so far (its confirmation was announced on May 6, 2005). On July 18, 2007 Anthe was announced. On March 6, 2008 it was announced that Cassini observations of a depletion of energetic electrons in Saturn's magnetosphere near Rhea might be the signature of a tenuous ring system around Saturn's second largest moon. On March 3, 2009, Aegaeon, a moonlet within the G Ring, was announced. Ground-based observations Study of Saturn's moons has also been aided by advances in telescopy. For the entire 20th century, Phoebe stood alone among Saturn's known moons in its highly irregular orbit. Beginning in 2000, three dozen additional irregular moons have been found using ground-based telescopes. A survey starting in late 2000 found thirteen new moons orbiting Saturn at a great distance in orbits that suggest they are fragments of larger bodies captured by Saturn's gravitational pull (Nature vol. 412, pp. 163–166). On May 3, 2005, astronomers using the Mauna Kea Observatory announced the discovery of twelve more small outer moons. On June 30, 2006 astronomers using the Subaru 8.2 m telescope announced the discovery of nine more small outer moons. On April 13, 2007 Tarqeq was announced On May 1, 2007 S/2007 S 2 and S/2007 S 3 were announced. Naming notes Some asteroids share the same names as moons of Saturn: 55 Pandora, 106 Dione, 577 Rhea, 1809 Prometheus, 1810 Epimetheus, 4450 Pan. See also Name conflicts of solar system objects. Groups Although the borders may be somewhat nebulous, Saturn's moons can be divided into ten groups. A-Ring and F-Ring moonlets In 2006, four tiny "moonlets" were found in Cassini images of the A Ring. Two larger moons had previously been spotted in the A Ring: Pan and Daphnis. These are large enough to clear gaps in the ring particles that circle the planet as they orbit Saturn. In contrast, the "moonlets" are only massive enough to clear a partial gap in the immediate vicinity of the moonlet itself; this wake is shaped like an airplane propeller and is only about ten km across.Cassini Finds 'Missing Link' Moonlet Evidence in Saturn's Rings The moonlets themselves are tiny, ranging from about 40–500 meters in diameter, and are too small to be seen directly. In 2007, the discovery of 150 more moonlets revealed that they are confined to three narrow bands (each a thousand kilometers wide, less than 1% the width of Saturn's rings) in the A Ring about 130,000 km from Saturn's center that are free from the disturbance of strong density waves, with the exception of two that have been seen outside the Encke Gap. (However, other areas of the A Ring without density-wave disturbances are apparently free of moonlets.) This suggests that they were formed from the breakup of larger bodies. It is estimated that the A Ring contains thousands of such fragments. Similar moonlets are thought to reside in the F Ring. There, "jets" of material may be due to collisions, initiated by perturbations from the nearby small moon Prometheus, of these moonlets with the core of the F Ring. One of the largest F-Ring moonlets may be the as-yet unconfirmed object S/2004 S 6. The F Ring also contains transient "fans" which are thought to result from even smaller moonlets, about 1-km in diameter, orbiting near the F Ring core. Ring shepherds Shepherd satellites are moons that orbit within, or just beyond, a planet's ring system. They have the effect of sculpting the rings: giving them sharp edges, and creating gaps between them. Saturn's shepherd moons are the moonlets, Pan, Daphnis, Atlas, Prometheus, Pandora, Aegaeon, in addition to the unconfirmed moons S/2004 S 4, S/2004 S 6 and S/2004 S 3. Co-orbitals Janus and Epimetheus are co-orbital moons. These two moons are of roughly equal size and have orbits with only a few kilometers difference in diameter, close enough that they would collide if they attempted to pass each other. Instead of colliding, however, their gravitational interaction causes them to swap orbits every four years. Inner large moons The innermost large moons of Saturn orbit within its tenuous E Ring. They are: *Mimas, the "Death Star" moon, with a deep crater one fifth its diameter; *Enceladus, a bright, striped moon whose geysers are the source of the E Ring; *Tethys, with a canyon running three-quarters its circumference that formed when an internal ocean froze; *Dione, with wispy terrain on its trailing hemisphere. Alkyonides These three moons orbit between Mimas and Enceladus: Methone, Anthe, and Pallene. Both Methone and Anthe possess faint ring arcs. A very faint ring has also been detected in Pallene's orbit. refhttp://ciclops.org/view/5253/CASSINI_IMAGES_RING_ARCS_AMONG_SATURNS_MOONS Trojan moons Trojan moons are another kind of co-orbital. Like the other co-orbitals, they are a feature unique to the Saturnian system. They are moons that orbit at exactly the same distance from Saturn as another moon, but at such a distance from the other moon that they never collide. Tethys has two tiny co-orbitals Telesto and Calypso, and Dione also has two, Helene and Polydeuces. All four of these moons orbit in the larger moons' L4 or L5 Lagrangian points, one in each point. Outer large moons Saturn's largest moons all orbit beyond its E Ring and can thus be considered a distinct group. They are: *Rhea, which may have its own ring system; *Titan, a huge moon, with methane lakes , sand dunes made of organic material, and an atmosphere thicker than Earth's; *Hyperion, a smallish, tumbling moon that looks like a sponge; *Iapetus, a two-toned, walnut-shaped moon with an equatorial ridge. Irregular moons Irregular moons are small satellites with large-radius, inclined, and sometimes retrograde orbits, believed to have been acquired by the parent planet through a capture process. They often occur as collisional families. The largest Saturnian irregular by far, and the only one known in any detail, is Phoebe. Inuit group The Inuit group are five prograde outer moons that are similar enough in their distances from Saturn and their orbital inclinations that they can be considered a group. They are Ijiraq, Kiviuq, Paaliaq, Siarnaq, and Tarqeq. Norse group The Norse group are 29 retrograde outer moons that are similar enough in their distance from Saturn to be considered a group. They are Aegir, Bergelmir, Bestla, Farbauti, Fenrir, Fornjot, Greip, Hati, Hyrrokkin, Jarnsaxa, Kari, Loge, Mundilfari, Narvi, Phoebe, Skathi, Skoll, Surtur, Suttungr, Thrymr, Ymir, S/2004 S 7, S/2004 S 12, S/2004 S 13, S/2004 S 17, S/2006 S 1, S/2006 S 3, S/2007 S 2, and S/2007 S 3. Gallic group The Gallic group are four prograde outer moons that are similar enough in their distance from Saturn and their orbital inclination that they can be considered a group. They are Albiorix, Bebhionn, Erriapus, and Tarvos. The diagram illustrates the orbits of the irregular satellites of Saturn discovered so far1. The eccentricity of the orbits is represented by the segments (extending from the pericentre to the apocentre) with the inclination represented on Y axis. The satellites above the axis are prograde, the satellites beneath are retrograde. The X axis is labelled in Gm (million km) and the fraction of the Hill sphere's (gravitational influence) radius (~65 Gm for Saturn). Prograde groups: Inuit and Gallic and the retrograde Norse group are clearly identifiable (from top to bottom). 1Named satellites are plotted in yellow; the unnamed satellites S/2004 Sxx (announced in 2005 and 2006) are plotted in white and S/2006 Sxx in grey. Table of moons The Saturnian moons are listed here by orbital period, from shortest to longest. Moons massive enough for their surfaces to have collapsed into a spheroid are highlighted in bold. The irregular captured moons (beyond Iapetus) are shaded light grey when prograde and darker grey when retrograde. Unconfirmed moons The following objects (observed by Cassini) have not been confirmed as solid bodies. It is not yet clear if these are real satellites or merely persistent clumps within the F Ring. Hypothetical moons These two moons were claimed to be discovered by different astronomers but never seen again. Since they both were said to orbit between Titan and Hyperion, it is possible that they are in fact identical, but most likely, none of these moons exist in reality. * Chiron which was supposedly sighted by Hermann Goldschmidt in 1861 but never observed by anyone else.Saturn's Ninth and Tenth Moons * Themis was allegedly discovered in 1905 by astronomer William Pickering, but never seen again. Nevertheless it was included in numerous almanacs and astronomy books until the 1960's.Hypothetical Planets See also * Saturn's moons in fiction * Natural satellites of Mars Jupiter Uranus Neptune Pluto * Timeline of discovery of Solar System planets and their natural satellites * Naming of natural satellites Notes References |name2=Spitale06 |ref2= }} |name3=Gazetteer |name4=Jacobson2006 |name5=Porco2007 }} External links *Saturn's Rings by NASA's Solar System Exploration *Saturn's Moons Astronomy Cast episode #61, includes full transcript. * Carolyn Porco: Fly me to the moons of Saturn Category:Moons of Saturn Category:Lists of moons af:Saturnus se natuurlike satelliete ast:Satélites de Saturnu be:Спадарожнікі Сатурна be-x-old:Спадарожнікі Сатурна bs:Saturnovi prirodni sateliti br:Loarennoù Sadorn (planedenn) bg:Естествени спътници на Сатурн ca:Satèl·lits de Saturn cs:Měsíce Saturnu da:Saturns måner de:Liste der Saturnmonde el:Δορυφόροι του Κρόνου es:Satélites de Saturno eo:Satelitoj de Saturno fr:Satellites naturels de Saturne ko:토성의 위성 hr:Saturnovi prirodni sateliti it:Satelliti naturali di Saturno he:ירחי שבתאי lv:Saturna pavadoņi lb:Lëscht vun de Mounde vum Saturn lt:Saturno palydovai hu:A Szaturnusz holdjai mk:Сателити на Сатурн ms:Satelit semulajadi Zuhal nah:Tzitzimicītlalli īmētz nl:Manen van Saturnus ja:土星の衛星と環 no:Saturns måner nn:Saturnmånane nds:List von de Saturn-Maanden pl:Księżyce Saturna pt:Satélites de Saturno ro:Sateliţii naturali ai lui Saturn ru:Спутники Сатурна simple:List of Saturn's moons sk:Mesiace Saturna sl:Saturnovi naravni sateliti fi:Saturnuksen kuut sv:Saturnus naturliga satelliter th:ดวงจันทร์ของดาวเสาร์ tr:Satürn'ün doğal uyduları uk:Супутники Сатурна zh:土星的卫星